Science in the spotlight

JET's ITER-Like Wall

December 2011

The installation of thousands of new tiles on the inside of the JET vessel (the ‘ITER-Like Wall') heralds a new experimental campaign on the world's largest fusion machine, which is hosted at CCFE. As scientists arrive from all over Europe eager to see how the machine will perform, here is an insight into what's behind the wall.

A new beginning

On 24 August 2011, a crowded JET control room, with an expectant air of excitement, witnessed the first plasma created on the machine for 22 months – a spectacular 1 Mega Amp pulse developed and lasted for 15 seconds. This was a remarkable achievement after a shutdown of nearly two years for the vacuum vessel and its new ITER-Like Wall (ILW).

A typical first plasma after an engineering break usually appears as a brief flash of light lasting less than a second. Was this longer pulse a testament to the materials of the new wall? Preliminary results have already confirmed the benefits of the new materials combinations which will be installed on the next global fusion experiment, ITER.

Focus on...the tiles

Over 2,000 bespoke tiles were installed during the shutdown, the vast majority using remote handling techniques. Some of the tiles were made in Huntsville, Alabama by Axsys Technologies, manufacturers of the mirrors for the planned successor to the Hubble space telescope – others came from 350 miles away, made by Atmostat in Paris. Each tile on the plasma facing wall weighs between 5-10 kg. The prominent tiles in direct contact with the plasma are made of pure beryllium and those recessed are inconel alloy coated with this light element. These were produced by Casting Technology International in Rotherham and coated with beryllium in Romania.

In the bottom of the vessel, where the exhaust system (divertor) is located the tile assemblies are faced with solid tungsten or tungsten coatings. These are crafted from pure tungsten on inconel carriers or from carbon fibre reinforced carbon coated with this hard, rare metal. Project Leader for the ILW, Dr Guy Matthews (pictured second from left with colleagues on the project) praises those involved in creating the new wall: “I hope they are all as proud as I am to have given JET a great experimental facility for the future.”

Choosing the materials

The new ILW is the first test of the material combination selected for the full performance phase of ITER. So why have beryllium and tungsten been chosen as the new plasma facing materials for fusion research replacing the previous Carbon Fibre Composite tiles? Not surprisingly, it's all about their properties. Beryllium is a very light metal which does not absorb tritium, which together with deuterium is one of the valuable fusion fuels. Light elements also have the least impact on the plasma temperature if they are eroded from the wall and this was another important consideration for ITER. Wall materials such as carbon are not as suitable as they absorb too much tritium and become too radioactive.

Being heat resistant up to 1,287 degrees Celsius means that berylium can be used in areas of the vessel where interactions with the plasma are significant but not the most intense. Tungsten has a higher melting point than its new counterpart on the ILW of 3,422 degrees C, making it an ideal choice to line the divertor where parts of the plasma do actually touch the wall. Here, the plasma is ‘diverted' down to a special area where temperatures of 1,000 degrees C area routine occurrence. As a heavy metal tungsten has a high atomic number and is also chosen for this area due its exceptional resistance to erosion from the hot plasma when contact occurs.

This combination of beryllium and tungsten being tested on JET is the same configuration planned for the lining of the ITER vacuum vessel under construction in Provence, Southern France.

The experimental campaign is underway

Scientists from throughout Europe will be participating in the new experimental campaign on JET, keen to learn how to operate the machine with its new wall. In addition to the ILW, during the recent JET shutdown other upgrades were made to the heating systems to provide more power and additional diagnostic devices were installed.

The research programme is co-ordinated by the European Fusion Development Agreement (EFDA), whose leader Dr Francesco Romanelli comments: “This is probably the largest effort that has been put into JET apart from the construction of the machine itself. With the expertise and contribution of many fusion laboratories, the JET team has succeeded in building a small ITER.” It is on this new ‘small ITER’ that a promising start has already been achieved with the creation of high purity plasmas established in ITER-relevant conditions – an encouraging sign for the use of these wall materials in ITER.

Studies so far have focused on the migration of Be from the plasma facing wall and the amount of fuel retained in the tiles. Other experiments have been examining the spectrum of light emitted by the plasma to learn about the role of impurities entering from the new tiles.

And so to the future…

The current ten-month experimental campaign will aim to verify that the wall materials chosen for ITER will behave as expected. This is crucial for its development as the ITER vessel will be ten times larger in volume and eventually run pulses for up to ten minutes for which the old carbon fibre composite tiles would not be suitable.The ILW will allow experiments to take place with ITER-relevant edge conditions, thus speeding up the early phases of ITER.

To date the recent pulses run on JET have used Ohmic heating – which involves heating the plasma solely by running an electric current through it. Other auxiliary heating methods, such as neutral beams will soon be used. In this way JET will progressively be brought up to higher power levels during the length of the campaign to allow a detailed investigation of the wall materials under conditions approaching those of ITER.

“The first experimental results look very promising although there is a long way to go,” says Dr Lorne Horton, Head of EFDA's JET Department (right). “Options for a further extension of JET's capabilities are being considered, in the context of strengthening the participation of other ITER parties in JET.”